The compound (6aR,9aS)-5,6a,7,8,9,9a-hexahydro-5-methyl-3-(phenylamino)-2-((4-(6-fluoropyridin-2-yl)phenyl)methyl)-cyclopent[4,5]imidazo[1,2-a]pyrazolo[4,3-e]pyrimidin-4(2H)-one is disclosed in WO 2009/075784 (U.S. Pub. No. 2010/0273754). This compound has been found to be a potent and selective phosphodiesterase 1 (PDE 1) inhibitor useful for the treatment or prophylaxis of disorders characterized by low levels of cAMP and/or cGMP in cells expressing PDE1, and/or reduced dopamine D1 receptor signaling activity (e.g., Parkinson's disease, Tourette's Syndrome, Autism, fragile X syndrome, ADHD, restless leg syndrome, depression, cognitive impairment of schizophrenia, narcolepsy); and/or any disease or condition that may be ameliorated by the enhancement of progesterone signaling. This list of disorders is exemplary and not intended to be exhaustive.
The publication WO 2009/075784 discloses (6aR,9aS)-5,6a,7,8,9,9a-hexahydro-5-methyl-3-(phenylamino)-2-((4-(6-fluoropyridin-2-yl)phenyl)methyl)-cyclopent[4,5]imidazo[1,2-a]pyrazolo[4,3-e]pyrimidin-4(2H)-one in free base form and generally in pharmaceutically acceptable salt form, but no specific salt was shown to have particular stability or desired properties. Because many pharmaceutical compounds can exist in different physical forms (e.g., liquid or solid in different crystalline, amorphous, polymorphous, hydrate or solvate forms) which can vary the stability, solubility, bioavailability or pharmacokinetics (absorption, distribution, metabolism, excretion or the like) and/or bioequivalency of a drug, it is of critical importance in the pharmaceutical development to identify a pharmaceutical compound of optimal physical form (e.g., free base or salt in solid, liquid, crystalline, hydrate, solvate, amorphous or polymorphous forms).